This invention relates to a device for supplying an offset preventing liquid to a fixing roller of a heating and fixing device of electrophotographic copying apparatus for preventing the occurence of a so-called offset phenomenon.
In the heating and fixing device, a powder image formed on a sheet is melted and becomes tacky. At this time, a portion or portions of such powder image may adhere to the fixing roller particularly when the fixing roller is in the form of a heating and fixing roller. As a result, such portion or portions will be transferred from the roller to a sheet which is supplied following the last previously supplied sheet to the heating and fixing device and become deposited thereon. This is what is generally referred to as an offset phenomenon.
This disadvantage can be eliminated by making the roller from a material which is free from the danger of causing the offset phenomenon to occur. No such material has, however, been discovered yet. Thus, it is usual practice nowadays to apply an offset preventing liquid to the surface of the roller as a means for preventing the occurrence of the offset phenomenon in electrophotographic copying apparatus.
FIG. 1 shows a prior art device for applying an offset preventing liquid to a heating and fixing roller which has been commonly in use. In the figure, the numeral 1 designates a heating and fixing roller coated on its outer periphery with Teflon (trade name) and maintained in pressing contact with a roller 2 which houses therein a heater (not shown). The two rollers rotate in opposite directions as indicated by arrows. An offset preventing liquid supplying cloth 4 immersed at one end portion in an offset preventing liquid 3 is maintained at the other end portion in contact with the outer periphery of the roller 1 so as to supply the offset preventing liquid 3 to the roller 1.
The device of the prior art shown and described hereinabove has the disadvantages of being cumbersome in construction and unable to supply a suitable amount of offset preventing liquid 3 in one operation to the heating and fixing roller 1, the amount delivered varying from one operation to another.
It has recently been discovered that the frequency of occurrence of the offset phenomenon can be lowered by coating the heating and fixing roller with slicone rubber which is superior to Teflon (trade name) in being less susceptible to the adhesion of particles forming a powder image in electrophotography. It is notable that it is possible completely to prevent the occurrence of the offset phenomenon by coating the heating and fixing roller with silicone rubber and applying thereto an offset preventing liquid, and that the amount of offset preventing liquid required for each operation can be greatly reduced.
This invention is based on the aforementioned discovery that the amount of offset preventing liquid used for each operation is very small when the heating and fixing roller is coated with silicone rubber.